You can buy special (expensive) K/N filter cleaner (I don't, I'm cheap)... I just soak it in a small tub of water with a bit of dish liquid... swish it around, let it soak for an hour, then allow to dry...
Filter oil, comes in a convenient spray can, and is usually colored so you can see that you have good coverage (worth the money for me, $6 or $7...)(I've been using the same can of Maxima Fab1 for over 5 years) ... It takes very little oil to do the job, (a can lasts forever)... just enough so that there are no dry spots and no more... once a year, or maybe twice if it's really dusty where you ride...
Any oil will work, but it isn't as sticky, and will need to be replaced more often... plus it's harder to get full coverage without putting on too much...
That's how I do it anyway...